protected <T> WindowedValue<T> makeWindowedValue(
      T output, Instant timestamp, Collection<? extends BoundedWindow> windows, PaneInfo pane) {
    final Instant inputTimestamp = timestamp;
    final WindowFn windowFn = windowingStrategy.getWindowFn();

    if (timestamp == null) {
      timestamp = BoundedWindow.TIMESTAMP_MIN_VALUE;
    }

    if (windows == null) {
      try {
        windows =
            windowFn.assignWindows(
                windowFn.new AssignContext() {
                  @Override
                  public Object element() {
                    throw new UnsupportedOperationException(
                        "WindowFn attempted to access input element when none was available"); // TODO: 12/16/15 aljoscha's comment in slack
                  }

                  @Override
                  public Instant timestamp() {
                    if (inputTimestamp == null) {
                      throw new UnsupportedOperationException(
                          "WindowFn attempted to access input timestamp when none was available");
                    }
                    return inputTimestamp;
                  }

                  @Override
                  public Collection<? extends BoundedWindow> windows() {
                    throw new UnsupportedOperationException(
                        "WindowFn attempted to access input windows when none were available");
                  }
                });
      } catch (Exception e) {
        Throwables.propagateIfInstanceOf(e, UserCodeException.class);
        throw new UserCodeException(e);
      }
    }

    return WindowedValue.of(output, timestamp, windows, pane);
  }
  private TriggerTester(
      WindowingStrategy<?, W> wildcardStrategy,
      ReduceFn<String, InputT, OutputT, W> reduceFn,
      Coder<OutputT> outputCoder)
      throws Exception {
    @SuppressWarnings("unchecked")
    WindowingStrategy<Object, W> objectStrategy = (WindowingStrategy<Object, W>) wildcardStrategy;

    this.windowFn = objectStrategy.getWindowFn();
    this.stubContexts = new StubContexts();
    this.outputCoder = outputCoder;
    executableTrigger = wildcardStrategy.getTrigger();

    this.runner =
        new ReduceFnRunner<>(
            KEY,
            objectStrategy,
            timerInternals,
            stubContexts,
            droppedDueToClosedWindow,
            droppedDueToLateness,
            reduceFn);
  }